1. Field of the Invention
This invention relates to an improved process for contaminant removal from hydrocarbons, and more specifically for the removal of nitrogen compounds from light hydrocarbons.
2. General Background
Nitrogen removal from petroleum stocks is widely practiced in the refining industry. Combined nitrogen may be undesirable from the standpoint of product quality or because nitrogen in fuels may increase emissions of nitrogen oxides during fuel combustion. Nitrogen removal often is dictated by the sensitivity to nitrogen of catalysts used in downstream processing units to this contaminant. Reforming catalysts usually are deactivated by the amounts of nitrogen found in cracked naphthas, for example, and heavy-oil feedstocks to selective hydrocracking catalysts may need treating for nitrogen reduction. Nitrogen removal from such stocks is effected by catalytic hydrotreating at elevated pressures and temperatures. Catalytic hydrotreating of light hydrocarbons, such as C.sub.3, C.sub.4, and C.sub.5 hydrocarbons, for nitrogen removal generally is not practiced due to high processing costs and concomitant loss of valuable light olefins which often are present in such streams.
Nitrogen compounds may be present in cracked, olefinic light-hydrocarbon streams as a result of the conversion of heavy nitrogen-containing molecules in the cracking feedstocks. Such nitrogen compounds found in these streams are removed to a large extent by water washing as would be practiced in many petroleum refineries. Conventional catalytic processing units which upgrade light olefinic hydrocarbons, such as polymerization and alkylation units, are not significantly affected by any remaining nitrogen.
New processing developments, however, are reducing the acceptable levels of nitrogen in light hydrocarbon streams. There is an increasing need for isobutene, isopentene and other tertiary-carbon olefins as chemical intermediates and for production of methyl t-butyl ether (MTBE), methyl t-amyl ether (TAME) and other ethers as gasoline components. MTBE and TAME in particular are experiencing rapid growth in demand as gasoline components, as restrictions on gasoline olefins and volatility force olefins out of gasoline and as ethers and alcohols are needed for reformulated gasolines with higher oxygen content. The need for branched-chain olefins as ether feedstocks increasingly exceeds the availability, and catalytic isomerization to alter the ratio of olefin isomers is one solution to this need.
Highly active and selective catalysts for olefin isomerization surprisingly have been found to be sensitive even to small quantities of weakly basic nitrogen compounds in the isomerization feed. Thus, nitrogen levels in light hydrocarbon streams that have been acceptable in catalytic processes in the past now must be reduced in order to effect olefin isomerization. Surprisingly in view of their relative boiling points, acetonitrile and propionitrile have been found in harmful quantities in light olefinic streams from FCC units. Conventional water washing and adsorption often do not result in sufficient removal of nitrogen compounds from hydrocarbon streams, and in addition, the nitrogen-containing streams from such removal processes present disposal problems. A problem facing workers in the art therefore is to provide technology to reduce nitrogen contaminants in light hydrocarbon streams below levels previously thought necessary for subsequent catalytic processing.
U.S. Pat. No. 5,191,146 discloses a process for the isomerization of pentenes or butenes using a highly selective non-zeolitic molecular-sieve catalyst, but does not suggest the need to remove nitrogen from the feestock.
The hydrolysis of nitriles to produce amides, and the further reaction of amides to form ammonia and carboxylic acids, are known in the art. U.S. Pat. No. 3,062,883 (Gilbert et al.) teaches a process for hydrolyzing nitriles in the presence of water and in the presence of a strongly basic resin to selectively yield the respective amides; using a styrene divinyl benzene copolymer substituted with tetramethyl ammonium hydroxide moieties as the resin, low temperatures avoid acid formation. U.S. Pat. No. Re. 28,525/3,381,034 (Greene et al.) discloses a process for hydrolyzing a variety of nitriles using a copper salt, which also may contain metallic copper, in the presence of water and optionally several other polar solvents; the products are relatively pure and easily recovered. U.S. Pat. No. 4,096,149 (Feldman et al.) teaches a method for the hydrolysis of a wide range of nitriles utilizing supported rhodium and a thiophosphite, with preferably mildly basic media based on water as the hydrolyzing agent.
Removal of nitrogen from hydrocarbons by adsorption using a zeolite is taught in U.S. Pat. No. 5,120,881 (Rosenfeld et al.) Copending application Ser. No. 08/031,795 teaches a combination for removal of sulfur and polar compounds from a C.sub.3 -C.sub.5 fraction, using adsorption to remove traces of polar compounds. Applicants believe that none of the art anticipates or suggests the invention described hereinbelow, which removes nitrogen compounds directly from light hydrocarbon streams while avoiding problems associated with adsorption processes such as finding a suitable desorbent and conversion or disposal of desorbed nitrogenous contaminants.